extreme ultraviolet lithography
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28-12-2009, 05:05 PM

.doc   EXTREME ULTRAVIOLET LITHOGRAPHY.doc (Size: 247 KB / Downloads: 195)

Silicon has been the heart of the world's technology boom for nearly half a century. Each year, manufacturers bring out the next great computer chip that boosts computing power and allows our Personal Computers to do more than we imagined just a decade ago. The current technology used to make microprocessors, deep ultraviolet lithography will begin to reach its limit around 2005. At that time, chipmakers will have to look to other technologies to cram more transistors onto silicon to create powerful chips. Many are already looking at extreme-ultraviolet lithography (EUVL) as a way to extend the life of silicon at least until the end of the decade.

Akin to photography, lithography is used to print circuits onto microchips Extreme Ultraviolet Lithography (EUVL) will open a new chapter in semiconductor technology. In the race to provide the Next Generation Lithography (NGL) for faster, more efficient computer chips, EUV Lithography is the clear frontrunner. Here we discusses the basic concepts and current state of development of EUV lithography (EUVL), a relatively new form of lithography that uses extreme ultraviolet (EUV) radiation with a wavelength in the range of 10 to 14 nanometers (nm) to carry out project and implimentationion imaging. EUVL is one technology vying to become the successor to optical lithography.


Silicon has been the heart of the world's technology boom for nearly half a century, but microprocessor manufacturers have all but squeezed the life out of it. The current technology used to make microprocessors will begin to reach its limit around 2005. At that time, chipmakers will have to look to other technologies to cram more transistors onto silicon to create more powerful chips. Many are already looking at extreme-ultraviolet lithography (EUVL) as a way to extend the life of silicon at least until the end of the decade.

Potential successors to optical project and implimentationion lithography are being aggressively developed. These are known as "Next-Generation Lithographies" (NGL's). EUV lithography (EUVL) is one of the leading NGL technologies; others include x-ray lithography, ion-beam project and implimentationion lithography, and electron-beam project and implimentationion lithography. Using extreme-ultraviolet (EUV) light to carve transistors in silicon wafers will lead to microprocessors that are up to 100 times faster than today's most powerful chips, and to memory chips with similar increases in storage capacity.


Extreme ultraviolet lithography (EUVL) is an advanced technology for making microprocessors a hundred times more powerful than those made today.

EUVL is one technology vying to replace the optical lithography used to make today's microcircuits. It works by burning intense beams of ultraviolet light that are reflected from a circuit design pattern into a silicon wafer. EUVL is similar to optical lithography in which light is refracted through camera lenses onto the wafer. However, extreme ultraviolet light, operating at a different wavelength, has different properties and must be reflected from mirrors rather than refracted through lenses. The challenge is to build mirrors perfect enough to reflect the light with sufficient precision


We know that Ultraviolet radiations are very shortwave (very low wavelength) with high energy. If we further reduce the wavelength it becomes Extreme Ultraviolet radiation. Current lithography techniques have been pushed just about as far as they can go. They use light in the deep ultraviolet range- at about 248-nanometer wavelengths-to print 150- to 120-nanometer-size features on a chip. (A nanometer is a billionth of a meter.) In the next half dozen years, manufacturers plan to make chips with features measuring from 100 to 70 nanometers, using deep ultraviolet light of 193- and 157-nanometer wavelengths. Beyond that point, smaller features require wavelengths in the extreme ultraviolet (EUV) range. Light at these wavelengths is absorbed instead of transmitted by conventional lenses


Computers have become much more compact and increasingly powerful largely because of lithography, a basically photographic process that allows more and more features to be crammed onto a computer chip.

Lithography is akin to photography in that it uses light to transfer images onto a substrate. Light is directed onto a mask-a sort of stencil of an integrated circuit pattern-and the image of that pattern is then project and implimentationed onto a semiconductor wafer covered with light-sensitive photoresist. Creating circuits with smaller and smaller features has required using shorter and shorter wavelengths of light.


The current process used to pack more and more transistors onto a chip is called deep-ultraviolet lithography, which is a photography-like technique that focuses light through lenses to carve circuit patterns on silicon wafers. Manufacturers are concerned that this technique might soon be problematic as the laws of physics intervene.

Intel, AMD, and Motorola have joined with the U.S. Department of Energy in a three-year venture to develop a microchip with etched circuit lines smaller than 0.1 micron in width. (Today's circuits are generally .18 micron or greater.) A microprocessor made with the EUVL technology would be a hundred times more powerful than today's. Memory chips would be able to store 1,000 times more information than they can today. The aim is to have a commercial manufacturing process ready before 2005.

Processors built using EUV technology are expected to reach speeds of up to 10 GHz in 2005-2006. By comparison, the fastest Pentium 4 processor today is 1.5 GHz.


Each year, manufacturers bring out the next great computer chip that boosts computing power and allows our personal computers to do more than we imagined just a decade ago. Intel founder Gordon Moore predicted this technology phenomenon more than 35 years ago, when he said that the number of transistors on a microprocessor would double every 18 months. This became known as Moore's Law.

Industry experts believe that deep-ultraviolet lithography will reach its limits around 2004 and 2005, which means that Moore's law would also come to an end without a new chipmaking technology. But once deep-ultraviolet hits its ceiling, we will see chipmakers move to a new lithography process that will enable them to produce the industry's first 10-gigahertz (GHz) microprocessor by 2007. By comparison, the fastest Intel Pentium 4 processor (as of May 2001) is 2.4 GHz. EUVL could add another 10 years to Moore's Law.

"EUV lithography allows us to make chips with feature sizes that are small enough to support 10 GHz clock speed. It doesn't necessarily make it happen," Don Sweeney, EUV Lithography program manager at Lawrence Livermore National Laboratory (LLNL), said. "The first thing we need to do is to make integrated circuits down to 30 nanometers, and EUV lithography will clearly do that." By comparison, the smallest circuit that can be created by deep-ultraviolet lithography is 100 nanometers.


Twenty five years ago, the computing equivalent of today's laptop was a room full of computer hardware and a cartload of punch cards. Since then, computers have become much more compact and increasingly powerful largely because of lithography

Why are smaller computer chips better and faster? It might seem a paradox, but as the size decreases, the chips become more powerful. It's as simple as getting to grandma's house faster if she lives next door rather than across town: the electronic signals zipping around the circuitry to solve computing problems have less distance to travel. Today's chip contains about 3,260 times more transistors than the chip of 1971.

A microprocessor -- also known as a CPU or central processing unit -- is a complete computation engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful -- all it could do was add and subtract, and it could only do that 4 bits at a time. But it was amazing that everything was on one chip.

The first microprocessor to make it into a home computer was the Intel 8080, a complete 8-bit computer on one chip, introduced in 1974. The PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. All of these microprocessors are made by Intel and all of them are improvements on the basic design of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster!

A microprocessor sometimes called a logic chip. It is the "engine" that goes into motion when you turn your computer on. A microprocessor is designed to perform arithmetic and logic operations that make use of small number-holding areas called registers. Typical microprocessor operations include adding, subtracting, comparing two numbers, and fetching numbers from one area to another. These operations are the result of a set of instructions that are part of the microprocessor design. When the computer is turned on, the microprocessor is designed to get the first instruction from the basic input/output system (BIOS) that comes with the computer as part of its memory. After that, either the BIOS, or the operating system that BIOS loads into computer memory, or an application program is "driving" the microprocessor, giving it instructions to perform. The following table helps you to understand the differences between the different processors that Intel has introduced over the years.
Name Date Transistors Microns Clock speed Data width MIPS
8080 1974 6,000 6 2 MHz 8 bits 0.64
8088 1979 29,000 3 5 MHz 16 bits
8-bit bus 0.33
80286 1982 134,000 1.5 6 MHz 16 bits 1
80386 1985 275,000 1.5 16 MHz 32 bits 5
80486 1989 1,200,000 1 25 MHz 32 bits 20
Pentium 1993 3,100,000 0.8 60 MHz 32 bits
64-bit bus 100
Pentium II 1997 7,500,000 0.35 233 MHz 32 bits
64-bit bus ~300
Pentium III 1999 9,500,000 0.25 450 MHz 32 bits
64-bit bus ~510
Pentium 4 2000 42,000,000 0.18 1.5 GHz 32 bits
64-bit bus ~1,700


In many respects, EUVL retains the look and feel of optical lithography as practiced today. For example, the basic optical design tools that are used for EUV imaging system design and for EUV image simulations are also used today for optical project and implimentationion lithography. Nonetheless, in other respects EUVL technology is very different from what the industry is familiar with. Most of these differences arise because the properties of materials in the EUV are very different from their properties in the visible and UV ranges.

Foremost among those differences is the fact that EUV radiation is strongly absorbed in virtually all materials, even gases. EUV imaging must be carried out in a near vacuum. Absorption also rules out the use of refractive optical elements, such as lenses and transmission masks. Thus EUVL imaging systems are entirely reflective. Ironically, the EUV reflectivity of individual materials at near-normal incidence is very low. In order to achieve reasonable reflectivityâ„¢s near normal incidence, surfaces must be coated with multilayer, thin-film coatings known as distributed Bragg reflectors. The best of these functions in the region between 11 and 14 nm. EUV absorption in standard optical photoresists is very high, and new resist and processing techniques will be required for application in EUVL.

Lithography is one of the key technologies that enable Intel to meet the challenge of Moore's Law by allowing a 30% decrease in the size of printed dimensions every two years. Intel has been an industry leader in advanced lithography with the early introduction of 248 nm and 193 nm lithography tools into high volume manufacturing. Intel is continuing this trend with strong investments in Extreme Ultraviolet (EUV) research at our Hillsboro, Oregon, and Santa Clara, California, sites.

The completion of the prototype machine (Engineering Test Stand) marks a major milestone for the program, since we have proven that EUV lithography works," said Chuck Gwyn, program manager of the EUV Limited Liability Company. "Our next step is to transfer the technology to lithography equipment manufacturers to develop beta and production tools."
Processors built using EUV technology are expected to reach speeds of up to 10 GHz in 2005-2006. By comparison, the fastest Pentium 4 processor today is 1.5 GHz. The prototype machine, called the Engineering Test Stand, was developed by industry-government collaboration among three U.S. Department of Energy national laboratories and a consortium of semiconductor companies called the EUV LLC. The consortium includes Intel Corporation, Motorola Inc., Advanced Micro Devices Inc., Micron Technology Inc., Infineon Technologies, and International The ETS was assembled at Sandia in Livermore, Calif. It will be used by LLC partners and lithography tool suppliers during the next year to refine the technology and get it ready to create a prototype commercial machine that meets industry requirements for high-volume chip production. The EUV LLC has developed relationships with more than 40 U.S.-based infrastructure companies to ensure that all of the key components can be attained for commercialization Business Machines


For describing the EUV chipmaking process we should have a clear idea of chipmaking process. Both are described in the following sections

Ultraviolet lithography can produce lines for integrated circuits as small as 39 nm in one recent test. To help sustain Moore's law and cram more and more gates and memory units into a given space, manufacturers of microchips must make the lines in their circuitry ever smaller. This usually means working with a shorter-wavelength light beam for creating the patterns used for inscribing fine features on silicon or metal surfaces. The form of lithography currently in mass production now can produce a half-pitch size (equal lines and spaces in between) of 90 nm and isolated line widths of 65 nm. To produce a later generation after that you would need even shorter wavelengths.
Silicon chips could be made more quickly and cheaply using a new technique developed by physicists in the US. Stephen Chou and colleagues at Princeton University have successfully imprinted patterns onto silicon using quartz moulds instead of the usual combination of lithography and etching. With a resolution of just 10 nm and an 'imprint time' of 250 ns, the new process could revolutionize the semiconductor industry - and keep 'Moore's Law' on track for another 25 years


Lithography is akin to photography in that it uses light to transfer images onto a substrate. In the case of a camera, the substrate is film. Silicon is the traditional substrate used in chipmaking. To create the integrated circuit design that's on a microprocessor, light is directed onto a mask. A mask is like a stencil of the circuit pattern. The light shines through the mask and then through a series of optical lenses that shrink the image down. This small image is then project and implimentationed onto a silicon, or semiconductor, wafer.

The wafer is covered with a light-sensitive, liquid plastic called photoresist. The mask is placed over the wafer, and when light shines through the mask and hits the silicon wafer, it hardens the photoresist that isn't covered by the mask. The photoresist that is not exposed to light remains somewhat gooey and is chemically washed away, leaving only the hardened photoresist and exposed silicon wafer.

The key to creating more powerful microprocessors is the size of the light's wavelength. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. More transistors equals a more powerful, faster microprocessor. That's the big reason why an Intel Pentium 4 processor, which has 42 million transistors, is faster than the Pentium 3, which has 28 million transistors.

As of 2001, deep-ultraviolet lithography uses a wavelength of 240 nanometers. A nanometer is one-billionth of a meter. As chipmakers reduce to 100-nanometer wavelengths, they will need a new chipmaking technology. The problem posed by using deep-ultraviolet lithography is that as the light's wavelengths get smaller, the light gets absorbed by the glass lenses that are intended to focus it. The result is that the light doesn't make it to the silicon, so no circuit pattern is created on the wafer.

This is where EUVL will take over. In EUVL, glass lenses will be replaced by mirrors to focus light. In the next section, you will learn just how EUVL will be used to produce chips that are at least five times more powerful than the most powerful chips made in 2001.

The components on a microchip are made by carving patterns into layers of doped and undoped silicon. In the standard technique, light is shone through a stencil onto a silicon wafer that is coated with a light-sensitive polymer known as a resist. Chemical etching then removes the regions of silicon coated with either the unexposed or the exposed polymer, until the desired structure is achieved. Finally, the remaining polymer is washed off.
But such 'photolithography' is expensive and complex, and the resolution of the technique is fast approaching the diffraction limit. This means that it will not be able to make features much smaller than the current minimum size of about 130 nm - and that the semiconductor industry could soon violate one of its guiding principles, The Moore's Law. Coined in 1965, this law described how the density of components on a chip doubled every 18 months, and was soon adopted by the semiconductor industry as a target.
Now Moore's Law could be back on track. Chou and co-workers say that their technique - known as laser-assisted direct imprint - can create features as small as 10 nm on silicon wafers. The new process also eliminates the need for the resist and washing steps


Here's how EUVL works:
1. A laser is directed at a jet of xenon gas. When the laser hits the xenon gas, it heats the gas up and creates plasma.

This source of extreme ultraviolet light is based on a plasma created when a laser is focused on a beam of xenon gas clusters expanding at supersonic speeds. (Besides invisible-to-the-eye extreme ultraviolet light, some visible light is also created, as seen in the blue glow in the photo.)

2. Once the plasma is created, electrons begin to come off of it and it radiates light at 13 nanometers, which is too short for the human eye to see.

Engineering Test Stand

3. The light travels into a condenser, which gathers in the light so that it is directed onto the mask.
4. A representation of one level of a computer chip is patterned onto a mirror by applying an absorber to some parts of the mirror but not to others. This creates the mask.
5. The pattern on the mask is reflected onto a series of four to six curved mirrors, reducing the size of the image and focusing the image onto the silicon wafer. Each mirror bends the light slightly to form the image that will be transferred onto the wafer. This is just like how the lenses in your camera bend light to form an image on film.

The ETS (Engineering Test Stand, also called prototype machine) includes a condenser optics box and a project and implimentationion optics box. Both boxes house complex optical trains of precision concave and convex spherical mirrors.

The conventional method for making the reflective masks for EUV lithography is called magnetron sputtering. But the defect rate for the process is about 10,000 defects per square centimeter, far too many for successful EUV lithography. The new process, embodied in Veeco's IBSD-350, produces precise, uniform, highly reflective masks with 81 alternating layers of molybdenum and silicon, each 3 to 4 nanometers thick. As the machine directs a beam of ions at the masks, the ions physically collide with each mask and form a vapor, which is precisely deposited on it at a defect density of less than 0.1 per square centimeter--a 100,000-fold improvement over conventional methods. This process also holds great promise for a number of other applications using virtually any material or combination of materials including metals, semiconductors, and insulators. A near-term possibility is making very-low-defect-density films for ultrahigh-density heads for the magnetic recording industry

The main role of the condenser optics box is to bring light to the reflective pattern on the mask. "We want to bring as much light to the mask and, ultimately, the wafer, as possible," explains Sweeney. "The more light we deliver, the shorter the exposure time. It's like taking a picture with a camera. A picture taken in bright noonday sun requires a shorter exposure time than does a picture of the same scene taken at twilight."

For the semiconductor industry, brighter EUV images mean shorter exposure times, which translate to manufacturing more chips at a faster rate. The optics design team from Lawrence Livermore and Sandia designed a condenser optics system that collects and transports a significant fraction of the EUV light from the source to the reflective mask.
Once the image is reflected from the mask, it travels through the project and implimentationion optics system. According to Sweeney, the project and implimentationion optics box is the optical heart of the lithographic exposure system. "It is to the system what an engine is to a car," he explains. The four mirrors of the ETS project and implimentationion optics system reduce the image and form it onto the wafer. "Again, imagine using a pocket camera. The camera lens transmits an image to the film, which-like the wafer-has a light-sensitive surface," says Sweeney.

The optics teams are now working on advanced designs for the project and implimentationion optics. They have a six-mirror design that promises to extend EUVL systems so that they can print features as small as 30 nanometers- a significant jump from the 70-nanometer limit of the ETS. According to Sweeney, extendability to smaller features is an important requirement for whatever lithographic technology the semiconductor industry finally decides

This wafer was patterned on a prototype device using extreme-ultraviolet lithography (EUVL).

This wafer was patterned on an integrated laboratory research system capable of printing proof-of-principle, functioning microelectronic devices using extreme ultraviolet lithography (EUVL). The EUV lithography research tool was assembled at Sandia National Laboratories in Livermore, Calif., which has joined with two other Department of Energy laboratories - Lawrence Livermore National Laboratory and Lawrence Berkeley National Laboratory - creating a Virtual National Laboratory to help develop EUV lithography for commercial use.

According to Sweeney, Deputy Program leader for Extreme Ultraviolet Lithography and Advanced Optics. In Lawrence Livermore National laboratory, California, the entire process relies on wavelength. If you make the wavelength short, you get a better image. He says to think in terms of taking a still photo with a camera.

"When you take a photograph of something, the quality of the image depends on a lot of things," he said. "And the first thing it depends on is the wavelength of the light that you're using to make the photograph. The shorter the wavelength, the better the image can be. That's just a law of nature."

As of 2001, microchips being made with deep-ultraviolet lithography are made with 248-nanometer light. As of May 2001, some manufacturers are transitioning over to 193-nanometer light. With EUVL, chips will be made with 13-nanometer light. Based on the law that smaller wavelengths create a better image, 13-nanometer light will increase the quality of the pattern project and implimentationed onto a silicon wafer, thus improving microprocessor speeds. This entire process has to take place in a vacuum because these wavelengths of light are so short that even air would absorb them. Additionally, EUVL uses concave and convex mirrors coated with multiple layers of molybdenum and silicon -- this coating can reflect nearly 70 percent of EUV light at a wavelength of 13.4 nanometers. The other 30 percent is absorbed by the mirror. Without the coating, the light would be almost totally absorbed before reaching the wafer. The mirror surfaces have to be nearly perfect; even small defects in coatings can destroy the shape of the optics and distort the printed circuit pattern, causing problems in chip function. Hence Before new lithography tools are even built, Chip makers must develop and demonstrate the necessary mask making capabilities.


Extreme Ultraviolet Lithography (EUVL) will open a new chapter in semiconductor technology. In the race to provide the Next Generation Lithography (NGL) for faster, more efficient computer chips, EUV Lithography is the clear frontrunner. At EUV Technology,

Successful implementation of EUVL would enable project and implimentationion photolithography to remain the semiconductor industry's patterning technology of choice for years to come. However, much work remains to be done in order to determine whether or not EUVL will ever be ready for the production line. Furthermore, the time scale during which EUVL, and in fact any NGL technology, has to prove itself is somewhat uncertain.

Several years ago, it was assumed that an NGL would be needed by around 2005 in order to implement the 0.1 um generation of chips. Currently, industry consensus is that 193nm lithography will have to do the job, even though it will be difficult to do so. There has recently emerged talk of using light at 157 nm to push the current optical technology even further, which would further postpone the entry point for an NGL technology. It thus becomes crucial for any potential NGL to be able to address the printing of feature sizes of 50 nm and smaller! EUVL does have that capability.


¢ http://www.intel.com
¢ http://www.howstuffworks.com
¢ http://www.sandia.com
¢ http://www.whatis.com
¢ http://www.euvl.com
¢ http://www.llnl.com


I express my sincere thanks to Prof. M.N Agnisarman Namboodiri ( Head of the Department, Computer Science and Engineering , MESCE ) and Mr. Zainul Abid ( Staff in charge ) for their kind co-operation for presenting the seminar and presentation.

I also extend my sincere thanks to all other members of the faculty of Computer Science and Engineering Department and my friends for their co-operation and encouragement.



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Extreme ultraviolet lithography

.docx   Extreme ultraviolet lithography.docx (Size: 76.65 KB / Downloads: 26)

EUVL light source
Neutral atoms or condensed matter cannot emit EUV radiation. For matter to emit it, Ionization must take place first. EUV light can only be emitted by electrons which are bound to multicharged positive ions; for example, to remove an electron from a +3 charged carbon ion (three electrons already removed) requires about 65 eV.[1] Such electrons are more tightly bound than typical valence electrons. The thermal production of multicharged positive ions is only possible in a hot dense plasma, which itself strongly absorbs EUV.[2][3] The Xe or Sn plasma sources for EUV lithography are either discharge-produced or laser-produced. Power output exceeding 100 W is a requirement for sufficient throughput. While state-of-the-art 193 nm excimer lasers offer intensities of 200 W/cm2,[4] lasers for producing EUV-generating plasmas need to be much more intense, on the order of 1011 W/cm2.[5] This indicates the enormous energy burden imposed by switching from generating 193 nm light (laser output approaching 100 W)[6] to generating EUV light (required laser or equivalent power source output exceeding 10 kW).[7]
A further characteristic of the plasma-based EUV sources under development is that they are not even partially coherent,[8] unlike the KrF and ArF excimer lasers used for current optical lithography. Further power reduction (energy loss) is expected in converting incoherent sources (emitting in all possible directions at many independent wavelengths) to partially coherent (emitting in a limited range of directions within a narrow band of wavelengths) sources by filtering (unwanted wavelengths and directions). On the other hand, coherent light poses a risk of monochromatic reflection interference and mismatch of multilayer reflectance bandwidth.[9]
As of 2008, the development tools had a throughput of 4 wafers per hour with a 120 W source.[10] For a 100 WPH requirement, therefore, a 3 kW source would be needed, which is not available in the foreseeable future. However, EUV photon count is determined by the number of electrons generated per photon which are collected by a photodiode; since this is essentially the highly variable secondary yield of the initial photoelectron, the dose measurement will be impacted by high variability. In fact, data by Gullikson et al.[11] indicated ~10% natural variation of the photocurrent responsivity. More recent data for silicon photodiodes remain consistent with this assessment.[12] Calibration of the EUV dosimeter is a nontrivial unsolved issue.[13] The secondary electron number variability is the well-known root cause of noise in avalanche photodiodes.[14]
If other problems are solved well enough to justify the investment, free electron lasers may provide the required light quality.

EUVL optics

EUVL is a significant departure from the deep ultraviolet lithography used today. All matter absorbs EUV radiation. Hence, EUV lithography needs to take place in a vacuum. All the optical elements, including the photomask, must make use of defect-free Mo/Si multilayers which act to reflect light by means of interlayer interference; any one of these mirrors will absorb around 30% of the incident light. This limitation can be avoided in maskless interference lithography systems. However, the latter tools are restricted to producing periodic patterns only.
The pre-production EUVL systems built to date contain at least two condenser multilayer mirrors, six project and implimentationion multilayer mirrors, and a multilayer object (mask). Since the optics already absorbs 96% of the available EUV light, the ideal EUV source will need to be sufficiently bright. EUV source development has focused on plasmas generated by laser or discharge pulses. The mirror responsible for collecting the light is directly exposed to the plasma and is therefore vulnerable to damage from the high-energy ions[15][16] and other debris.[17] This damage associated with the high-energy process of generating EUV radiation has precluded the successful implementation of practical EUV light sources for lithography.
The wafer throughput of an EUVL exposure tool is a critical metric for manufacturing capacity. Given that EUV is a technology requiring high vacuum, the throughput is limited (aside from the source power) by the transfer of wafers into and out of the tool chamber, to a few wafers per hour.[18]
Another aspect of the pre-production EUVL tools is the off-axis illumination (at an angle of 6 degrees)[19] on a multilayer mask. The resulting asymmetry in the diffraction pattern causes shadowing effects which degrade the pattern fidelity.[20]
EUVL's shorter wavelength also increases flare, resulting in less than perfect image quality and increased line width roughness.[21]
Heating per feature volume (e.g., 20 nm cube) is higher per EUV photon compared to a DUV photon, due to higher absorption in resist. In addition, EUV lithography results in more heating due to the vacuum environment, in contrast to the water cooling environment of immersion lithography.
Heating is also a particularly serious issue for multilayer mirrors used, because EUV is absorbed within a thin distance from the surface. The heating density is higher. As a result, water cooling is expected to be used for the high heating load; however, the resulting vibration is a concern.[22]
A recent study by NIST and Rutgers University found that multilayer optics contamination was highly affected by the resonant structure of the EUV mirror influencing the photoelectron generation and secondary electron yield.[23]
Since EUV is highly absorbed by all materials, even EUV optical components inside the lithography tool are susceptible to damage, mainly manifest as observable ablation.[24] Such damage is a new concern specific to EUV lithography, as conventional optical lithography systems use mainly transmissive components and electron beam lithography systems do not put any component in the way of electrons, although these electrons end up depositing energy in the exposed sample substrate.

EUV exposure of photoresist

When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter.[25] It has been estimated that about 4 secondary electrons on average are generated for every EUV photon, although the generation volume is not definite.[26] These secondary electrons have energies of a few to tens of eV and travel tens of nanometers inside photoresist (see below) before initiating the desired chemical reaction. This is very similar to the photoelectron migration for the latent image formation in silver halide photographic films. A contributing factor for this rather large distance is the fact that polymers have significant amounts of free volume.[27] In a recent actual EUV print test,[28] it was found 30 nm spaces could not be resolved, even though the optical resolution and the photoresist composition were not the limiting factor.

Initial distribution of reactive species after EUV absorption. Molecules are excited and ionized within a few nanometers from the absorption point, and electrons are thermalized within 20 nanometers from the absorption point. The inset picture shows the multispur effect, where several electron-ion pairs generated by the EUV photon may interact with one another.
In particular, for photoresists utilizing chemical amplification for higher throughput:[29][30]
e- + acid generator -> anion -> dissociated anion products
This reaction, also known as "electron attachment" or "dissociative electron attachment" is most likely to occur after the electron has essentially slowed to a halt, since it is easiest to capture at that point. The cross-section for electron attachment is inversely proportional to electron energy at high energies, but approaches a maximum limiting value at zero energy.[31] On the other hand, it is already known that the mean free path at the lowest energies (few to several eV or less, where dissociative attachment is significant) is well over 10 nm,[32][33] thus limiting the ability to consistently achieve resolution at this scale. In addition, electrons with energies < 20 eV are capable of desorbing hydrogen and fluorine anions from the resist,[34] leading to potential damage to the EUV optical system.[35]
EUV photoresist images often require resist thicknesses roughly equal to the pitch.[36] This is not only due to EUV absorption causing less light to reach the bottom of the resist but also to forward scattering from the secondary electrons (similar to low-energy electron beam lithography). Conversely, thinner resist transmits a larger fraction of incident light allowing damage to underlying films, yet requires more dosage to achieve the same level of absorption.
Since the photon absorption depth exceeds the electron escape depth, as the released electrons eventually slow down, they dissipate their energy ultimately as heat.
An EUV dose of 1 mJ/cm2 generates an equivalent photoelectron dose of 10.9 μC/cm2. Current demonstration doses exceed 10 mJ/cm2, or equivalently, 109 μC/cm2 photoelectron dose.
The use of higher doses and/or reduced resist thicknesses to produce smaller features only results in increased irradiation of the layer underneath the photoresist. This adds another significant source of photoelectrons and secondary electrons which effectively reduce the image contrast. In addition, there is increased possibility of ionizing radiation damage to the layers below.
The extent of secondary electron and photoelectrons in blurring the resolution is dependent on factors such as dose, surface contamination, temperature, etc.

EUVL defects

EUVL faces specific defect issues analogous to those being encountered by immersion lithography. Whereas the immersion-specific defects are due to unoptimized contact between the water and the photoresist, EUV-related defects are attributed to the inherently ionizing energy of EUV radiation. The first issue is positive charging, due to ejection of photoelectrons[37] freed from the top resist surface by the EUV radiation. This could lead to electrostatic discharge or particle contamination as well as the device damage mentioned above. A second issue is contamination deposition on the resist from ambient or outgassed hydrocarbons, which results from EUV- or electron-driven reactions.[38] A third issue is etching of the resist by oxygen,[39] argon or other ambient gases, which have been dissociated by the EUV radiation or the electrons generated by EUV. Ambient gases in the lithography chamber may be used for purging and contamination reduction. These gases are ionized by EUV radiation, leading to plasma generation in the vicinity of exposed surfaces, resulting in damage to the multilayer optics and inadvertent exposure of the sample.[40]
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extreme ultraviolet lithography

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A brief history

Nearly all of today’s electronic devices rely on key internal semiconductor components, known as integrated circuits (ICs). ICs are manufactured through a critical process known as lithography, which is the determining factor in keeping pace with the quest of the electronics industry to shrink ICs and other related products even more.
Lithography is a patterning method that creates an IC layout on a resist layer of a silicon wafer or other semiconducting substrate. It mainly consists of three parts: a) the pattern printer, b) photoresist technology, and c) the mask fabrication.
Lithography technology was introduced to the semiconductor industry when ICs were invented in 1958. The original lithography used light of the visible g-line (436 nm) and the ultraviolet i-line (365 nm), which was easily produced with a mercury arc lamp. With the progress of technology and the reduction of the feature size, the wavelength of the exposure light had to be reduced several times. When the IC feature size was reduced to about half a micron (500 nm), the g-line and the i-line could no longer be used, and therefore deep ultraviolet 248 nm KrF and 193 nm ArF excimer lasers were introduced. Currently, the 193 nm lithography combined with immersion and double patterning technology is the state of the art.

The map of worldwide research

Since 1988, many studies on EUVL have been conducted in North America, Europe, and Asia, through state sponsored programs, industrial consortiums, and individual companies.
In the early and mid-1990s, systematic research was mainly performed by the Lawrence Livermore National Laboratory (LLNL), Sandia National Laboratory (SNL), and Lawrence Berkeley National Laboratory (LBNL), as well as AT&T Bell Laboratories and several universities. In 1997, an industrial consortium, the EUV LLC, was formed by Intel, Motorola, and Advanced Micro Device (AMD), to continue work on EUVL. At the same time, the Virtual National Laboratories (VNL) was also formed by LLNL, SNL, and LBNL to conduct a program sponsored by EUV LLC.
In Europe, an industrial consortium, the Extreme Ultraviolet Concept Lithography Development System (EUCLIDES), was formed in 1998 by ASM Lithography (ASML), Carl Zeiss, and Oxford Instruments. Since then, EUVL studies in Europe have made significant progress, with ASML leading.

The challenges
To date, no “showstoppers” have been identified, but challenges are present in almost every aspect of EUVL technology. Some challenges are common to all NGL technologies, e.g. resist resolution and line-edge roughness (LER). Other challenges are unique to EUVL, e.g. resist outgassing owing to the EUVL high-vacuum environment. In the past 20 years the main topics of research in EUVL have been: source, optics, mask, multilayer coating, resist, metrology, reticle handling, defects, and contamination control.
Today, commercial alpha lithography step-and-scan tools are installed with full field capability; EUVL power at intermediate focus (IF), however, has not yet met the target of 180 watt intermediate focus (IF) power required for volume manufacturing. EUV IF power has been improving gradually from xenon to tin discharge-produced plasma (DPP), or to laser-produced plasma (LPP).

The opportunities
EUVL was originally planned in 1988 for the 100 nm technology node. However, the extension of optical lithography delayed the adoption of EUVL and other NGL technologies. In 1997, implementation was predicted for the 65 nm node. A further extension of optical lithography reduced the predicted EUVL implementation to under the 45 nm node. The immersion exposure technology combined with the double patterning method delayed EUVL implementation further. At the moment it is predicted that EUVL will have some pilot-scale applications at the 32 nm technology node or will be used in full production for the 22 nm half-pitch technology node.

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